Performance evaluation of microbial desulphurization of Waterberg steam coal as a pre-combustion technique

摘要

Coal is considered as one of the major energy sources in South Africa. Ninety-one percent (91%) of the electricity generated in South Africa is based on coal combustion in steam-turbine driven power plants (IEA, 2014). In the process of electricity generation, coal is transformed into ash and gases, which cause numerous environmental problems like acid rain. Environmental impacts associated with the use of coal as a primary source for electricity generation and development of new technologies to mitigate the resulting emissions has received extensive attention recently. South African National Power Utility's (Eskom) has made the decision to employ flue gas desulphurization (FGD) technology for power stations being constructed and where possible to retrofit on the existing power stations (Makgato and Chirwa, 2017). The FGD technology is used to reduce sulphur emissions in coal-fired power utilities by using pulverised limestone in a spray tower to react with SO_2 in the flue gas and remove sulphur as a solid product (gypsum). In spite of the good performance of the FGD technology thus far, the process produces effluent with high concentrations of the cationic and anionic impurities as well as heavy metals. Hence, FGD must be compared with other pre-combustion technologies in in modern power plants in order to consistently meet the SO_2 emission standards and increase power station availability. As already mentioned, there are effective technologies to remove sulphur after or during combustion, but there is no technology for removing sufficient sulphur before combustion to meet the minimum emissions standards. One method of minimising the later problem is to reduce the amount of pollutant elements in coal, such as sulphur, before it is burnt. This will reduce the amount of sulphur-derived gases like SOx formation and acid formation after those gases leaving the boiler utility (Weerasekara et al., 2008).